Re: [PATCH v9 06/13] PCI: liveupdate: Preserve bus numbers during Live Update

[Pranjal Shrivastava <praan@google.com>]

On Fri, Sep 18, 2026 at 08:06:32PM +0000, David Matlack wrote:
> Keep the secondary and subordinate bus numbers that the previous kernel
> programmed into bridges, rather than assigning new ones, if the previous
> kernel preserved any device across a Live Update. Do this even on
> architectures that would otherwise always assign bus numbers themselves,
> e.g. when pci=assign-busses is passed.
> 
> Preserved devices must be allowed to continue performing memory
> transactions across a Live Update, so the kernel cannot change the
> fabric topology. Changing the bus numbers of a bridge changes the
> RequesterIDs of the devices below it, which would require disabling and
> flushing any in-flight memory transactions first.
> 
> Apply the policy globally rather than only to the paths that contain
> preserved devices. Bus numbers have to be preserved above a preserved
> device anyway, since an upstream bridge cannot expand its window. A
> global policy matches the scope of pcibios_assign_all_busses(), and
> gives an answer that cannot change part way through the two passes of a
> bridge scan.
> 
> Bridges that do not have bus numbers are still assigned new ones, so
> hot-adding a bridge keeps working, both during and after a Live Update.
> The two-pass bridge scan guarantees such bridges are only assigned bus
> numbers above those already claimed by preserved bridges.
> 
> The exception is a bridge that was preserved but comes up without a
> valid bus number configuration, e.g. because it was reset during kexec.
> Refuse to assign it new bus numbers, since that would silently change
> the BDF of every preserved device in its hierarchy. Also refuse to
> assign bus numbers to the other bridges on the same bus, since the bus
> numbers of the failed bridge can no longer be read from hardware and
> handing them out would let an unrelated device inherit the BDF of a
> preserved device.
> 
> Require that CONFIG_CARDBUS is not enabled to enable
> CONFIG_PCI_LIVEUPDATE since preserving bus numbers on PCI-to-CardBus
> bridges requires additional work but is not a priority at the moment.
> 
> Signed-off-by: David Matlack <dmatlack@google.com>

Reviewed-by: Pranjal Shrivastava <praan@google.com>

Thanks,
Praan